1. Field of the Invention
The invention relates to a new method for estimating the organic matter content of sedimentary rocks and principally host-rocks from data recorded in wells or bore-holes by well-logging probes.
The method of the invention concerns more particularly the determination of the organic matter content of host-rocks by using data obtained from at least two logs of different kinds, such for example as a sound log and a resistivity log. In the first case, it will be recalled, a probe is lowered into a bore-hole having sound wave transmitting and receiving transducers and the propagation speed of these waves in the formations crossed by the bore-hole and in the vicinity thereof is measured. In the second case, it is the electric resistivity of the ground surrounding the well which is measured.
Estimation of the organic matter content of host-rocks forms an important element in the oil-bearing estimation of a sedimentary basin.
2. Description of the Prior Art
The methods used for estimating the organic matter content of sedimentary rocks is derived from organic geochemistry. They consist essentially in quantifying and characterizing the organic matter contained in samples taken from well cuttings or else possibly from geological core samples, when they are available. The geochemical data obtained is generally scant and fragmented because of the lack of available samples. The organic matter content of geological formations is therefore very often only known in a restricted number of depths and the estimation thereof is related to the representativity of the cuttings taken from the wells. Each sample taken from the cuttings is often representative of a certain thickness of ground (of the order of several meters) and in formations with rapid facies variations it does not give a fine enough picture of their heterogeneity. Furthermore, some samples are not very significant to the extent that they were taken from zones which have been mixed with falls coming from upper layers. In addition, the presence of certain products such as gasoil or fuel oil, which may be in the drilling sludge, completely falsifies the measurements made in the laboratory.
It should also be noted that corrections must be made to the depths at which the samples are taken, measured by the drillers, so that the data resulting from the geochemical analyses may be usefully compared with those obtained by well-logging and this requires resetting of depths.
The estimation of the values of the organic matter content all along a well is therefore generally time-consuming and very delicate.
Research has already been undertaken to try to link the organic matter content to measurements made by well-logging probes of very different types. They have shown that the measurements were influenced by the different components of the rock and particularly by its organic matter content. But up to now little success has been obtained in defining the specific influence of the organic matter content on the well-logging measurements, with a sufficient accuracy to make a correct and representative estimation thereof possible.
The methods already used have an empiric character and are applicable only in particular cases.